Mineral sands may contain many valuable minerals, among which are principally ilmenite, rutile, zircon, leucoxene, monazite and gold. These minerals are extracted by using differences in density and differences in the magnetic and electrical properties of the individual mineral species to separate them from the less valuable mineral components of the sands, and from each other.
Several prior art techniques are available for the separation of mineral sands into their valuable components. The most common method is generalized in FIG. 1 in block diagram form. The mineral sands are delivered as a wet raw sand to a gravity circuit (WET PLANT) to produce a coarse heavy mineral concentrate (HMC). This HMC may then be fed to a second stage where the magnetic properties of some of the component minerals are used to effect a further separation and concentration.
Ilmenite is a composite of iron and titanium oxides and is weakly magnetic. Highly magnetic minerals, such as magnetite, are removed from the HMC by a low intensity magnetic separator. The residual material may then be subjected to a wet high intensity magnetic separation (WHIMS) stage to concentrate the ilmenite. The WHIMS product may then be processed through an electrostatic stage in a DRY MILL.
The compound of particular interest for which ilmenite is the principal source is titanium dioxide, and the typical titanium dioxide concentration when the above prior art process is applied to ilmenite from the West Coast of The South Island of New Zealand ranges between 45%-47% TiO.sub.2 with typical assays of silicon dioxide (silica) in the range of 4% to 6% and dialuminium trioxide (alumina) of 2% to 2.5%. By contrast, concentrates of West Australian ilmenites commonly contain TiO.sub.2 in excess of 50%.
Due to the presence of iron oxides in ilmenite, the magnetic susceptibility of ilmenite can be increased by roasting under a variety of conditions. This increase in magnetic susceptibility is a well-known phenomenon and occurs through alteration of the chemical composition and crystalline structure, for example as discussed in the articles referred to below and allows the ilmenite to be readily separated from other minerals for example chromite, quartz, garnet and rutile, etc. by magnetic separation techniques.
One such prior art process is that operated by the Richards Bay Minerals (RBM) Company in South Africa which mines and treats raw sands which are high in chromite to recover ilmenite and other minerals. The raw sands are first processed through gravity and WHIMS circuits. The WHIMS separates the feed into non-magnetic and magnetic fractions, and the non-magnetic fraction, which contains rutile and zircon is then treated in a DRY MILL after being separated from the magnetic ilmenite/chromite fraction. The ilmenite/chromite fraction is roasted with excess oxygen at about 800.degree. C. for 40 minutes. This magnetizes the ilmenite and allows it to be separated magnetically from the chromite as described at pp. 555-8 of "Magnetic Methods for the Treatment of Minerals", by J. Svoboda, Elsevier (1987), or Australian Patent 502866.
Another process is described in GB 2043607 which describes roasting an ilmenite ore in an hydrous atmosphere to enhance its magnetic susceptibility to separate it from rutile as an "impurity".
Besides the above patents, articles describing magnetising roasting known to the applicant are by Curnow & Parry (Nature, Dec. 11, 1954, p. 1101, Journal and Proc. of the Royal Society of N.S.W. Vol. 89 [1955] p. 64), Ishikawa and Akimoto (Journal of Physical Society of Japan Vol. 12 No. 10, Oct. 1957; Vol. 13, No. 10, Oct. 1958) and Bozorth, Walsh & Williams (Physical Review Vol. 108, No. 1, Oct. 1, 1957, p. 1083).
The process described by Curnow & Parry is one of oxidation in air at temperatures between 600.degree. C. and 800.degree. C. A ferric to ferrous ratio of 1.3 is achieved while prolonged roasting in excess of 800.degree. C. produces only a weakly ferromagnetic resultant. This is much the same as the Richards Bay process.
Ishikawa describes using temperatures of 1100.degree. C. for up to 12 hours and quenching to produce a solid solution of xFeTiO.sub.3 (1-x)Fe.sub.2 O.sub.3 with maximal magnetic properties when 1.0&gt;x&gt;0.5. Ishikawa is also referred to in Bozorth et. al. which is concerned with the magnetization of ilmenite at low temperatures.
Ilmenite deposits are found in many countries for example South Africa, United States of America, Australia, India, New Zealand and other areas of the world. The ilmenite deposits in various countries and locations can differ in their compositions.
In particular the ilmenite found in the South Island of New Zealand contains abundant inclusions and selvedges of silicate minerals. Metallurgically these inclusions have the effect of lowering the magnetic susceptibility and conductivity of grains of ilmenite containing inclusions, while enhancing the content of silica and alumina and other deleterious compounds in an ilmenite concentrate with a consequent relative depletion of the titanium dioxide content. Such composite grains can be difficult to separate magnetically or electrostatically, and can result in lower than average yields and higher than average capital and direct operating costs than are usual in the mineral sands industry.
The South Island of New Zealand ilmenites also occur in common association with abundant garnet. The garnet has a specific gravity and size range close to that of the ilmenite and this also creates problems in the first stage of gravity separation in the known processes. The magnetic susceptibility and conductivity of this garnet are also close to those of the ilmenite such that the employment of the known separation stages is costly while the loss of ilmenite from the process is also high.
Because the silicate inclusions give significant "inbuilt" levels of silica and alumina in a slag or synthetic rutile feedstock, it is important to remove discrete crystals such as garnet, quartz or other deleterious silicate minerals in the mineral dressing process. The conventional mineral dressing process as shown in FIG. 1 can remove nearly all the unwanted discrete minerals from a West Coast South Island of New Zealand mineral sand but at the cost of an overall recovery ranging from 65% to 75% of the ilmenite. The best ilmenite concentrate that can be achieved may contain from about 1% to 2% of discrete silicate minerals and will assay approximately 46.5% to 47% titanium dioxide. When this concentrate is processed in an electric arc smelting furnace it can provide, according to FIG. 3, an equivalent of approximately 73%-83% titanium dioxide in slag, depending on the level of iron (FeO) in the slag acceptable in the slag-making process and to the consumer.